9 Joab said to Amasa, "Is it well with you, my brother?" And Joab took Amasa by the beard with his right hand to kiss him. 10 But Amasa did not notice the sword in Joab's hand; Joab struck him in the belly so that his entrails poured out on the ground, and he died. He did not strike a second blow. Then Joab and his brother Abishai pursued Sheba son of Bichri.
Matthew Henry's Commentary on 2 Samuel 20:9-10
Commentary on 2 Samuel 20:4-13
(Read 2 Samuel 20:4-13)
Joab barbarously murdered Amasa. The more plot there is in a sin, the worse it is. Joab contentedly sacrificed the interest both of the king and the kingdom to his personal revenge. But one would wonder with what face a murderer could pursue a traitor; and how, under such a load of guilt, he had courage to enter upon danger: his conscience was seared.